Optically transparent films useful for covering food product containers and packaging food products are ideally formulated to provide consumers with a clear view of the packaged food product while maintaining its appearance and freshness. Films with oxygen transmission rate (OTR) control properties extend the shelf life of refrigerated freshly cut produce and fresh meat since the presence of either too much or too little oxygen in the package lowers shelf life. For example, in the packaging of lettuce, excessive oxygen in the package results in enzymatic browning of cut surfaces of the lettuce. On the other hand, insufficient oxygen in the package results in lettuce spoilage caused by anaerobiosis. Similarly, fresh meat packaging permits delivery of sufficient oxygen to the myoglobin contained within the meat to maintain its natural red color and thus achieve the fresh looking product preferred by consumers.
Packaging films made from polyvinyl chloride (PVC) and plasticizer have been used to wrap red meat and produce. PVC films provide a sparkling clear view of the packaged product, and, when formulated with plasticizers, exhibit cling and stretch properties. However, it is often desirable to wrap the food product on a tray, heat seal the film around the tray and heat shrink the film. Conventional PVC packaging films have difficulty forming hermetic seals because PVC has a narrow sealing range. The precision required to seal conventional PVC films is too high for manufacturing with conventional packaging equipment.
Packaging films made from olefinic materials can be formulated to control OTR, to seal to themselves and to be heat shrinkable at lower temperatures than conventional PVC films. However, films of olefinic materials are susceptible to surface moisture buildup in the form of small beads that obscure a consumer's view of the product contained in the package. Reducing fog (surface moisture buildup) in olefinic films by adding standard anti-fog agents has been largely unsuccessful. Anti-fog agents are difficult to contain on the film surface because they tend to wash off. Anti-fog agents also interfere with other film surface properties of the olefinic film making it difficult to print and seal.
In other packaging technology areas, films for applications such as contact packaging for bottles or base films for pharmaceutical packaging have used blends of conventional PVC, meaning non-crosslinked PVC resin, and conventional PVC/VA, meaning non-crosslinked PVC/VA resin. For example, U.S. Pat. No. 6,265,041 discloses a film for use as a cover sheet in pharmaceutical packaging. The disclosed film comprises between 1–80% by weight of a PVC resin and 20–99% by weight of a PVC/VA resin. U.S. Pat. No. 4,264,010 discloses a shrink wrap film for contact packaging products, such as a large bottle for carbonated beverages. The disclosed film comprises PVC and 5 to 20 parts by weight PVC/VA.
While the disclosed films comprise PVC/VA and PVC, these films require no controlled OTR and contain only small amounts of plasticizer. For example, in the film disclosed in U.S. Pat. No. 6,265,041, the amount of plasticizer is less than 2% by weight of the resin portion of the film. U.S. Pat. No. 6,265,041 discloses examples of film containing ESO, a plasticizer, in amounts ranging from 1.5 parts by weight out of 114.5 parts of resin composition to about 2.0 parts by weight out of 113 parts of resin composition. Stated in percentages by weight, these amounts range from 1.3% by weight of the resin composition to 1.8% by weight of the resin composition.
For the examples of films with PVC and PVC/VA blends disclosed in U.S. Pat. No. 4,264,010, the amount of plasticizer ranges from 11 to 17 weight parts out of 119.7 parts to 141.7 parts. Stated in percentage by weight, the amount of plasticizer in these examples ranges from about 9% to about 12% by weight of the film.
While homogenous blends of conventional PVC and conventional PVC/VA may have been achieved with these low levels of plasticizer, it has generally been believed that blending highly plasticized PVC with a copolymer such as PVC/VA was too difficult to manufacture commercially. PVC resins are typically blended with a high speed impeller, which creates heat by friction. PVC/VA fuses at a lower temperature than PVC, so PVC/VA will tend to agglomerate and fuse together at the temperatures required to blend PVC. If the PVC/VA forms agglomerates, the PVC/VA agglomerate may discolor when exposed to heat during processing, forming black impurities in any film formed from the blend.
It would be desirable, therefore, to have available a film that is optically transparent and has a controllable OTR. A film that is heat shrinkable and that may be hermetically sealed to itself by conventional packaging equipment is also needed.